204 lines
9.2 KiB
HTML
204 lines
9.2 KiB
HTML
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<title>Generate A Changeset From A Session Object</title>
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<a href="intro.html"><h2>Session Module C Interface</h2></a><h2>Generate A Changeset From A Session Object</h2><blockquote><pre>int sqlite3session_changeset(
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sqlite3_session *pSession, /* Session object */
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int *pnChangeset, /* OUT: Size of buffer at *ppChangeset */
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void **ppChangeset /* OUT: Buffer containing changeset */
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);
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</pre></blockquote><p>
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Obtain a changeset containing changes to the tables attached to the
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session object passed as the first argument. If successful,
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set *ppChangeset to point to a buffer containing the changeset
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and *pnChangeset to the size of the changeset in bytes before returning
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SQLITE_OK. If an error occurs, set both *ppChangeset and *pnChangeset to
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zero and return an SQLite error code.</p>
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<p>A changeset consists of zero or more INSERT, UPDATE and/or DELETE changes,
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each representing a change to a single row of an attached table. An INSERT
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change contains the values of each field of a new database row. A DELETE
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contains the original values of each field of a deleted database row. An
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UPDATE change contains the original values of each field of an updated
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database row along with the updated values for each updated non-primary-key
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column. It is not possible for an UPDATE change to represent a change that
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modifies the values of primary key columns. If such a change is made, it
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is represented in a changeset as a DELETE followed by an INSERT.</p>
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<p>Changes are not recorded for rows that have NULL values stored in one or
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more of their PRIMARY KEY columns. If such a row is inserted or deleted,
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no corresponding change is present in the changesets returned by this
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function. If an existing row with one or more NULL values stored in
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PRIMARY KEY columns is updated so that all PRIMARY KEY columns are non-NULL,
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only an INSERT is appears in the changeset. Similarly, if an existing row
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with non-NULL PRIMARY KEY values is updated so that one or more of its
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PRIMARY KEY columns are set to NULL, the resulting changeset contains a
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DELETE change only.</p>
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<p>The contents of a changeset may be traversed using an iterator created
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using the <a href="../session/sqlite3changeset_start.html">sqlite3changeset_start()</a> API. A changeset may be applied to
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a database with a compatible schema using the <a href="../session/sqlite3changeset_apply.html">sqlite3changeset_apply()</a>
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API.</p>
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<p>Within a changeset generated by this function, all changes related to a
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single table are grouped together. In other words, when iterating through
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a changeset or when applying a changeset to a database, all changes related
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to a single table are processed before moving on to the next table. Tables
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are sorted in the same order in which they were attached (or auto-attached)
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to the sqlite3_session object. The order in which the changes related to
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a single table are stored is undefined.</p>
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<p>Following a successful call to this function, it is the responsibility of
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the caller to eventually free the buffer that *ppChangeset points to using
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<a href="../c3ref/free.html">sqlite3_free()</a>.</p>
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<p><h3>Changeset Generation</h3></p>
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<p>Once a table has been attached to a session object, the session object
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records the primary key values of all new rows inserted into the table.
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It also records the original primary key and other column values of any
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deleted or updated rows. For each unique primary key value, data is only
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recorded once - the first time a row with said primary key is inserted,
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updated or deleted in the lifetime of the session.</p>
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<p>There is one exception to the previous paragraph: when a row is inserted,
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updated or deleted, if one or more of its primary key columns contain a
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NULL value, no record of the change is made.</p>
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<p>The session object therefore accumulates two types of records - those
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that consist of primary key values only (created when the user inserts
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a new record) and those that consist of the primary key values and the
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original values of other table columns (created when the users deletes
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or updates a record).</p>
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<p>When this function is called, the requested changeset is created using
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both the accumulated records and the current contents of the database
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file. Specifically:</p>
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<p><ul>
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<li> For each record generated by an insert, the database is queried
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for a row with a matching primary key. If one is found, an INSERT
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change is added to the changeset. If no such row is found, no change
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is added to the changeset.</p>
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<p> <li> For each record generated by an update or delete, the database is
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queried for a row with a matching primary key. If such a row is
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found and one or more of the non-primary key fields have been
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modified from their original values, an UPDATE change is added to
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the changeset. Or, if no such row is found in the table, a DELETE
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change is added to the changeset. If there is a row with a matching
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primary key in the database, but all fields contain their original
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values, no change is added to the changeset.
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</ul></p>
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<p>This means, amongst other things, that if a row is inserted and then later
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deleted while a session object is active, neither the insert nor the delete
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will be present in the changeset. Or if a row is deleted and then later a
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row with the same primary key values inserted while a session object is
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active, the resulting changeset will contain an UPDATE change instead of
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a DELETE and an INSERT.</p>
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<p>When a session object is disabled (see the <a href="../session/sqlite3session_enable.html">sqlite3session_enable()</a> API),
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it does not accumulate records when rows are inserted, updated or deleted.
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This may appear to have some counter-intuitive effects if a single row
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is written to more than once during a session. For example, if a row
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is inserted while a session object is enabled, then later deleted while
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the same session object is disabled, no INSERT record will appear in the
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changeset, even though the delete took place while the session was disabled.
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Or, if one field of a row is updated while a session is disabled, and
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another field of the same row is updated while the session is enabled, the
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resulting changeset will contain an UPDATE change that updates both fields.
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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